COMPLEMENTARY STRANDS MELT APART AND REANNEAL
The complementary antiparallel strands of DNA form an elegant molecule that is able to unzip or melt and come back together or reanneal (Fig. 3.9). The hydrogen bonds that
hold the two halves together are relatively weak. Heating a sample of DNA will dissolve the hydrogen bonds, resulting in two complementary single strands. If the same sample of DNA is slowly cooled, the two strands will reanneal so that G matches with C and A matches with T, as before.
The proportion of G-C base pairs affects how much heat is required to melt a double helix of DNA. G-C base pairs have three hydrogen bonds to melt, whereas A-T base pairs have only two. Consequently, DNA with a higher percentage of GC will require more energy to melt than DNA with fewer GC base pairs. The GC ratio is defined as follows:
The ability to zip and unzip DNA is crucial to cellular function, and has also been exploited in biotechnology. Replication and transcription rely on strand separation to generate either new DNA or RNA strands, respectively. In molecular biology research, many techniques, from PCR to library screening, exploit the complementary nature of DNA strands.
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